The present invention relates to a pressure regulating valve, and more particularly to a pressure regulating valve primarily for use with a fuel injection device for an automotive internal combustion engine.
FIG. 1 shows a known fuel injection system for an automotive internal combustion engine. As shown in FIG. 1, a pressure regulating valve 2 is provided upstream of a fuel injection valve 1 to maintain an absolute fuel pressure at a value higher than an absolute intake manifold pressure by a fixed value (normally 2.55 kg/cm.sup.2). A fuel pressure to be hereinafter referred means a gauge pressure obtained by subtracting an atmospheric pressure from the absolute fuel pressure, and an intake manifold pressure to be hereinafter referred means a vacuum obtained by subtracting the absolute intake manifold pressure from the atmospheric pressure. The fuel injection system shown in FIG. 1 includes a fuel tank 3, a fuel pump 4, a fuel filter 5, a fuel damper 6 and a surge tank 7. A suction air is induced in the direction of arrow as shown. A vacuum passage 8 is provided to induce a vacuum in the surge tank 7 into the fuel regulating valve 2. A fuel passage 9 is provided to communicate the fuel pump 4 with the fuel injection valve 1 and the pressure regulating valve 2. A fuel return passage 10 is provided to return an excessive fuel to the fuel tank 3.
FIG. 2 is a vertical sectional view of the conventional pressure regulating valve 2. A diaphragm 13 is fixed between upper and lower casings 11 and 12 to define a diaphragm chamber 14 in the upper casing 11 and a fuel chamber 15 in the lower casing 12. A diaphragm spring 16 is inserted in the diaphragm chamber 14, and a spring force of the diaphragm spring 16 is set to a value such that the absolute fuel pressure is regulated to be always higher than the absolute intake manifold pressure by a fixed value (normally 2.55 kg/cm.sup.2) by means of a pressure regulating mechanism to be hereinafter described. The casing 11 is connected with a vacuum connection pipe 17 communicated through the vacuum passage 8 to the surge tank 7 (see FIG. 1). The casing 12 is connected at its side portion with a fuel inlet connection pipe 18 communicated through the fuel passage 9 to the fuel injection valve 1 and the fuel pump 4 (see FIG. 1). The casing 12 is also connected at its axial portion with a fuel outlet connection pipe 19 in such a manner that the upper end portion of the fuel outlet connection pipe 19 projects into the casing 12 and is provided with a seat member 20. A valve member 21 is mounted at a central portion of the diaphragm 13 to open and close the fuel outlet by the reciprocation of the diaphragm 13.
In operation, when the diaphragm 13 is urged upwardly by the fuel pressure against the spring force (2.55 kg/cm.sup.2), an excessive fuel is discharged from the fuel outlet connection pipe 19 and is returned through the fuel return passage 10 to the fuel tank 3. At this time, since the pressure in the diaphragm chamber 14 is equal to that in the surge tank 7, the fuel pressure is fluctuated with a change in the intake manifold vacuum. Therefore, the fuel pressure is so controlled as to satisfy the relation that the fuel pressure is equal to the fixed value minus the intake manifold vacuum. The fixed value is adjusted by the spring force of the diaphragm spring, and the latter is set so that the fixed value is usually 2.55 kg/cm.sup.2. (Japanese Patent Publication No. 49-37049, for example)
As a fuel for the automotive internal combustion engine, a low-boiling point fuel such as alcohol containing fuel and high reed vapor pressure fuel has been recently utilized. Such a low-boiling point fuel generates a vapor at a fuel temperature of about 80.degree. C. Particularly, in the case of parking the vehicle under the blazing sun for a long time, the temperature of fuel in the fuel pipe is greatly increased to cause the generation of fuel vapor.
Such vaporization of the fuel at high temperatures can be prevented by increasing a fuel pressure. As to the low-boiling point fuel as mentioned above, the fuel pressure should be set to 3.5 kg/cm.sup.2 at the fuel temperature of about 80.degree. C. in order to prevent the vaporization.
However, when the spring force of the diaphragm spring 16 is set to balance the fuel pressure of 3.5 kg/cm.sup.2, a discharge rate of the motor driven fuel pump 3 is decreased. If the discharge rate of the fuel pump 3 is intended to be increased under this condition, a current flowing in the fuel pump 3 must be increased. As a result, wear of a brush and a commutator of a d.c. motor is increased, causing a problem in durability of the motor driven fuel pump 3.